Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for removing seismic survey interference in a marine environment and, more particularly, to mechanisms and techniques for selecting a pilot signal for driving the seismic sources of a current seismic survey to minimize the impact of other seismic sources of another seismic survey that takes place in a vicinity of the current seismic survey.
Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of geophysical structures under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where the oil and gas reservoirs are located.
Reflection seismology is a method of geophysical exploration which can be used to generate the image of the subsurface. Marine reflection seismology is based on the use of a controlled source that sends pressure waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, distributed close or on the earth's surface, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
In many areas, for example, the Gulf of Mexico, where many seismic contractors operate at the same time, it is common to find independent seismic surveys being conducted in adjacent areas. In order to reduce the interference (cross-talk) between the two seismic surveys, a time-share agreement is typically negotiated so that one crew is inactive while the other shoots. The restrictions imposed by time-share agreements cut productivity in half and greatly impact crew's costs.
Currently, one of the most common seismic sources in marine applications is the airgun, which is essentially impulsive (e.g., compressed air is suddenly allowed to expand). The frequency content of impulsive sources is not fully controllable, and different impulsive sources are selected depending on the needs of a particular survey. The use of impulsive sources can pose certain safety and environmental concerns.
Another class of sources that may be used is vibratory sources, for example, see U.S. Pat. No. 8,830,794, the entire content of which is incorporated herein. For vibratory sources, the source signal excitation is typically a sweep, a swept frequency sine wave excitation signal over a pre-determined sweep bandwidth for a predetermined time interval. Other excitation signals are possible, for example, band limited pseudorandom signals like those disclosed in U.S. Pat. No. 8,619,497. The output of the seismic source may be continuous or semi-continuous.
The source array emits a sweep over a given sweep length as it is towed by a moving vessel. Typically, after some instrument reset period and/or listen time, the sweep is repeated to start a new recording for the new source/receiver position. Thus, a typical raw record includes both the sweep and listen time. Correlation or source signature deconvolution may be employed to collapse the data to produce a record that is similar to what might be obtained using an impulsive source. The technique of using a vibratory source followed by correlation to collapse the data is called Vibroseis. For the case of continuous or semi-continuous emissions, signals may be recorded continuously or semi-continuously over long time intervals. The recordings may be parsed later, during the processing phase, to divide the mother record up into smaller daughter records that each span a smaller time interval (for example, about 20 s in length). The daughter records time spans may overlap or not.
A seismic survey system 100 having a source array that includes individual vibratory sources is illustrated in FIG. 1. This figure shows a source array 103 being towed behind a vessel 101, at a shallow depth, following a survey course 119 operating within permit block 121. Source array 103 may include one or more source elements 107 (e.g., a vibratory element). When source array 103 is activated, acoustic energy is coupled into the water and transmitted into the earth, where part of the energy is partially reflected back from the ocean bottom and from rock formation interfaces (rock layer that has a change in acoustic impedance). Streamers 117, which are also towed by vessel 101, include seismic sensors 109, which are used to record the reflected energy. Head floats 105 are attached to the heads of the streamers for maintaining them at a desired depth. A seismic sensor may include hydrophones, geophones, accelerometers or other sensors or combinations of these sensors. Streamer 117 can be fluid filled or encapsulated with a solid fill material. Other source arrays (not shown) may be towed by different source vessels and they may be offset from source array 103 and used to generate acoustic signals whose reflections are also recorded by streamers 117.
A second vessel 151, whose course 161 is offset/different from the course 119 of the first vessel 101, may tow another source array 153. Source array 153 may include either impulsive or vibratory sources 159 and operate independently/asynchronously with respect to source array 103. In addition, source vessel 151 may tow corresponding streamers 163.
While vessel 101, source array 103 and streamers 117 belong to a first seismic survey 100, vessel 151, source array 153 and streamers 163 belong to a second seismic survey 150. For distinguishing these elements from each other, vessel 101 would be referred to as first survey vessel, vessel 151 would be referred to as second survey vessel, source array 103 would be referred to as first survey source array, and so on. The first and second survey vessels may be more than 10 km apart. Second survey vessel 151 may operate in permit block 171, which may be different than permit block 121 as illustrated in FIG. 1, or have overlapping sections.
Depending upon the seismic sensor type, the returning energy is recorded as a pressure, velocity or acceleration variation as a function of time at each sensor's position. Combining recordings made at multiple source and sensor locations can be used to form an image of the subterranean features of the earth.
Seismic signals that fall within the frequency range of 1-150 Hz are most important for seismic imaging today. Because signals from the second seismic survey 150 nearby will generally have similar spectral content as the signals recorded at the first seismic survey 100, cross-talk issues will be present in both seismic surveys. Cross-talk issues include, but are not limited to, recording at a same seismic receiver signals from sub-surfaces corresponding to both the first and second surveys. When this happens, separating the signals from the different seismic surveys becomes a difficult and expensive task.
In this regard, FIG. 2 illustrates the geometry of the cross-talk problem. The first seismic survey 100 operates first survey source array 103 while the second seismic survey 150 operates the second survey source array 153, with a separation distance 201 between them. Both source arrays 103 and 153 operate at modest depth (e.g., 5 to 50 m) below water surface 225. In the Gulf of Mexico, for example, separation distance 201 must exceed 30 km during the entire survey. Streamer 117 includes, as discussed above, many seismic sensors. However, for simplicity, FIG. 2 shows only three hydrophone groups 213, 215 and 217. Acoustic waves 203 are emitted by source array 103. A portion of the energy associated with waves 203 propagates to seafloor 205 to illuminate the subterranean survey target. Rays 207, 209, and 211 represent the total acoustic signal received by hydrophone groups 213, 215 and 217 due to source array 103. These rays may correspond to subterranean reflections due to changes in rock layer properties, reflections from the seafloor itself or other means like direct arrivals and/or surface reflections and/or refractions. These rays carry information about subsurface 219.
Simultaneously, source array 153 emits acoustic waves 225 that strike the seafloor 227, with some of that energy reflecting off as waves 229, 231, and 233, which are being also recorded by hydrophone groups 213, 215 and 217. However, the waves carry information about a different subsurface 235. If the angle of incidence for wave front 225 and region 227 is large (60 degrees or more), then the reflection coefficient can approach unity, especially if the seafloor is hard. In other cases, where the angle of incidence is smaller, the interfering signal from source array 153 may be from refracted energy that entered the earth at one point and then reemerges at second point, where it reenters the water and reaches the hydrophones. Irrespective if the mechanism is controlled by reflection or refraction, typically the cross-talk from the second survey is the largest noise contributor to the first survey. Thus, the seismic data recorded at the hydrophone groups 213, 215 and 217 describe two different sub-surfaces 219 and 235, which is undesirable.
For the scenario illustrated in FIG. 2, the recorded data from hydrophone groups 213, 215 and 217 is a superposition of desirable acoustic signals from paths 207, 209 and 211 and undesirable cross-talk signals following paths 229, 231 and 233. For the case where the second seismic survey is a competitor using vibratory sources, neither the sweep signal of the source may be known, nor the precise location of the source and shot times.
If the other crew's source signal is unknown, it may prove difficult to estimate their signal and remove it from the recorded seismic data. Methods like blind deconvolution may be employed, but may not prove fruitful and may require a large processing effort.
Thus, it would be valuable and desirable for one seismic survey to have a method that would allow independent seismic surveys to be conducted simultaneously with another seismic survey.